An actin-like protein from amoebae of dictyostelium discoideum

An actin-like protein has been isolated and purified from amoebae of Dictyostelium discoideum. The 3.7S protein polymerizes upon addition of 0.1 m KCl to a polymer of 26S. An increase in viscosity accompanies this polymerization and electron micrographs have revealed beaded, helical filaments with a diameter of 60–75 Å and an axial periodicity of 350 Å. These F-actin-like filaments produced a 5-fold activation of muscle myosin Mg-ATPase at low ionic strength. When incubated with rabbit muscle heavy meromyosin (HMM) the amoeba F-actin-like protein formed typical “arrowhead” structures with polarized binding of HMM and arrowhead spacings of 350 Å. In SDS polyacrylamide disc gel electrophoresis the purified amoeba protein migrates as a single band corresponding to a molecular weight of 48,000 daltons. The amino acid composition is very similar to that of muscle actin and includes the unusual amino acid 3-methylhistidine.